1. Field of the Invention
The present invention relates generally to telecommunications between earth transmitting-receiving stations via a satellite.
2. Description of the Prior Art
More exactly, the invention relates to a method of allocating communication channels to calling stations and called stations in a satellite telecommunications network employing frequency re-use in response to a call setting-up request transmitted by the calling station to a control station SC. As shown in FIG. 1, a satellite telecommunication network is typically defined by a plurality N=9 of earth transmitting-receiving stations S1 through S9, the control station SC and a satellite SA. Setting up a call between a calling station, such as the station S3, and a called station, such as the station S6, of the plurality of stations entails allocating a channel or frequency band B3 to the calling station S3 and a channel B6 to the called station S6, each of the channels B3 and B6 belonging to a given beam. The data to be transmitted produced by the station S3 is transmitted to the satellite SA on the uplink channel B3 and retransmitted from the satellite SA to the called station S6 on the downlink channel B6.
A call setting-up phase is started at the initiative of the calling station S3 which produces a call setting-up request message intended for the control station SC via the satellite SA. This call setting-up request message is sent on an uplink signalling channel BS from the calling station to the satellite and retransmitted on a downlink signalling channel BS from the satellite to the control station SC. The control station SC retransmits a call setting-up message to the calling station S3 as soon as two channels, the channels B3 and B6 in this example, can be respectively allocated to the stations S3 and S6.
The prior art provides two methods of managing radio beam coverage in a satellite network in the context of a low-power satellite SA.
In a first method, the network having a predetermined number of channels, a variable number of channels are allocated to each beam according to the level of traffic. To this end dynamic reallocation of channels of the payload of the satellite SA between the radio beams is effected, depending on the traffic demand on those beams. In a method of this kind the beams are fixed, which means that the coverage of each beam for each of the channels constituting it has a constant extent defined by an antenna gain at less equal to a minimal gain. On the other hand, for a given beam, the number of channels varies in time. Reducing by one the number of channels in one beam entails increasing by one the number of channels in another beam. A satellite implementing this method is called a capacity reallocation matrix satellite.
A second method is described in U.S. Pat. No. 5,355,138 issued on Oct. 11, 1994, the contents of which are hereby incorporated by way of reference into this description. A satellite network is managed by coverage reconfiguration. In this second method, unlike the first method previously mentioned, the number of channels in each radio beam is fixed and the coverage of each beam is modified in time by reducing or increasing the coverage area in accordance with call setting-up requests produced in the network. In practise, although the number of channels in each beam is constant, there is provision for exchanging channels between beams. Allocating a channel of a first beam to a second beam entails allocation to the first beam of a channel of the second beam. Accordingly, the "bandwidth" of the set of channels of a beam remains constant. The size of the beam coverage is reconFig.d in the following manner.
To set up a call, i.e. a link, between a calling station and a called station of the network via the satellite SA, the calling station first sends a call setting-up request message to the control station SC via a signalling channel BS of the satellite SA. The development in the configuration of the network relative to the calling and called stations is simulated in the control station. A call setting-up authorization message is transmitted by the control station SC to the calling station when simulated authorizations have been derived by the control station SC for both the calling and called stations. Each of these simulated authorizations corresponds to the possible allocation of a respective free channel for the calling station and the called station.
Two main scenarios are provided for each station in the simulation in the control station. Each of these two scenarios is directly related to the limited power of the satellite SA.
In the first scenario (FIG. 5 of U.S. Pat. No. 5,355,138) relating to the situation in which the calling or called station does not belong to any radio beam coverage, the coverage nearest the station is selected first. The beam coverages other than the selected coverage are then reduced in size to free up a fraction of the power output by the satellite SA in these other beams. This freed up portion of the power is then used to increase the surface (footprint) of the selected coverage in order to include the station in it. A simulated authorization is derived in the control station as soon as the station can be included in the selected coverage by increasing its surface and a channel is free in the selected beam coverage.
In the second scenario (FIG. 11 of U.S. Pat. No. 5,355,138), relating to the situation in which the calling or called station is included in a radio beam coverage, there are two sub-scenarios. Either the beam of this coverage has a free channel, in which case this free channel is allocated to the station; or there is no free channel in the beam associated with the coverage including the station, in which case the coverage nearest this coverage having a free channel is looked for. A load transfer is then effected between the nearest coverage having a free channel and the coverage including the station. This load transfer entails increasing the surface of the nearest coverage in order to include the station in it and commensurate reduction in the surface of the coverage including the station. A simulated authorization is derived for the station as soon as it can be included in the adjacent coverage and a channel is free in the adjacent coverage.
As soon as simulated authorizations have been derived in the control station SC for both the calling and called stations, the control station sends to the satellite SA power and phase-shift control values to modify the geometries of the coverages concerned, that is to say, also, those including the calling and called stations in accordance with the simulation carried out. A call setting-up authorization message is also sent via the satellite SA to the calling station in order that a communication phase between the calling and called stations can begin.
In a variant of the first scenario, relating to the situation in which the station is not included in any coverage, there is provision for selecting not only the nearest coverage but also the lowermost surface coverage in the network and for each of these two coverages to reduce the surfaces of the other coverages in order to include the station in the selected coverage, either the nearest one or the one with the lowermost surface. In this variant, two respective gains for the station are calculated by simulation in the control station SC according to whether the station is included in the nearest coverage or the lowermost surface coverage. The coverage to include the station S is chosen as that which offers the highest gain. The power and phase-shift control values for the radiating elements of the satellite antenna are thus transmitted by the control station SC to the satellite SA.
The first and second prior art methods described hereinabove ignore frequency re-use in the network and therefore the problem caused by management of channels corresponding to the same frequency band in different beams. For reasons relating to limitation of the frequency bandwidth available on the satellite, it can be beneficial to re-use the same channels or frequency bands in different beams.
Nevertheless, it seems that such re-use of the same band of frequencies in different beams causes interference dependent on the angular separation between the different beams.